Permanent-magnet synchronous motor

ABSTRACT

In a combination of, for example, a rotor having eight magnetic poles and a stator having twelve slots and pulsating components of permeance, which generate a sinusoidal cogging torque having maxima of the same number as the number of poles of the rotor, pressurizing parts arranged in predetermined positions applying a force at an outer periphery of the stator directed inwardly to cancel the pulsating components of the cogging torque.

TECHNICAL FIELD

The present invention relates to a permanent-magnet type synchronousmotor contributory to reduction in cogging torque, and a method ofmanufacturing the same.

BACKGROUND ART

Cogging torque in an inner rotor type permanent-magnet type synchronousmotor comprises pulsating components in torque, which is generatedbetween teeth of a stator core (stator iron core) and a magnet rotor(rotor) when the magnet rotor is rotated by an external drive at thetime of non-current-carrying to a winding, and only an order of a leastcommon multiple of the number 2 p of magnetic poles of a rotor magnetand the number Z of teeth (slots) of a stator core appears theoretically(see Non-Patent Document 1). However, this theory is limited to the casewhere rotors (mainly, magnets) and stator cores are uniform in shape andmaterial properties and manufactured completely symmetrically withrespect to the number of poles and the number of slots.

Since the number of poles and the number of slots get out of symmetricproperty in real machines, in particular, in a field of volumeproduction, however, components of cogging torque having lower ordersthan the order of the least common multiple appear in many cases atlarge amplitudes. An increase in cogging torque has a great influenceupon performance of products because of causing degradation inpositioning accuracy for servomotors and degradation in feeling ofsteering for motors for automotive power steering.

Returning to the principle of torque generation, an explanation will begiven to a mechanism for generation of pulsating components in torque.Torque is related to magnetic flux density and increases in the casewhere magnetic flux is easy to pass. Easiness, with which magnetic fluxpasses, is called permeance (reciprocal of magnetic resistance), andtorque is generated in proportion to the square thereof. Accordingly,when permeance is varied, cogging torque is generated. In the case wherea magnet making a source of generation of magnetic flux involves anonuniform distribution and a symmetric property inconsistent with thenumber of poles, these are sensed on a side of a stator and pulsationhaving orders consistent with the number of slots and higher ordercomponents thereof appears. Higher order components are composed ofhigher harmonic wave components, which nonuniform components do notnecessarily make a near-sinusoidal wave change to thereby cause.

Since main magnetic flux passes through an air from a magnet to returnto the magnet through a back yoke portion from teeth of a stator core,materials of passages are divided into two. One is an air which ispresent between a rotor and a stator, and the other is a magnetic bodythat makes a core (iron core) In recent years, a laminate of flat rolledmagnetic steel sheets and strip is in many cases used for such magneticbody, the magnetic property of flat rolled magnetic steel sheets andstrip causes a problem frequently. In the case where flow of mainmagnetic flux involves a nonuniform distribution and a symmetricproperty inconsistent with the number of slots, these are sensed on aside of a rotor and pulsation having orders consistent with the numberof poles and higher order components thereof appears.

For passages of magnetic flux, magnetic permeability μ indicative ofeasiness, with which magnetic flux passes, is constant in an air, sothat a quantity of magnetic flux in the air appears due to a change inlength of gaps (air gaps). Physical quantities having an influence onmain magnetic flux moving through a rotor magnet and stator teeth areroughly classified into two, one of which is a gap (called an air gap)indicative of a shortest distance between an outside diameter of therotor magnet and the stator teeth, and the other of which is a gap(generally called an open width) between the adjacent stator teeth.While an air gap is determined by an outside diameter shape of a rotorand an inside diameter shape of a stator, the inside diameter shape of astator causes a problem in many cases.

Also, in the case where in order to facilitate a winding process, amethod, in which a core is partially or wholly divided between teeth, isemployed instead of manufacturing a stator core from a substantiallycircular-shaped unitary core, minute clearances are present to makeclearance gaps when the divided portions are joined together.

Also, in the case where a core is partially divided and joined afterwinding, for convenience of a joint process, such part is in some casesmade different in structure from the remaining part wherebynon-uniformity in structure is generated.

Subsequently, cores manufactured from a magnetic body such as flatrolled magnetic steel sheets and strip generate, in many cases,individual differences in magnetic permeability due to various factorsand a nonuniform distribution in the same individual one. One of thefactors for generation of individual differences is a composition(grade) of flat rolled magnetic steel sheets and strip being a corematerial. Also, one of the factors for generation of differences in thesame individual one is due to those different magnetic permeabilities inspecific portions of a core obtained by a method of punching a coreshape, which are caused by a difference (called magnetic anisotropy) inmagnetic property between a direction of rolling of, for example, flatrolled magnetic steel sheets and strip and a direction perpendicularthereto. Also, when flat rolled magnetic steel sheets and strip arepunched by blades of a metallic die, forces exerted by the blades causedegradation of teeth end surfaces in magnetic permeability, and theprocess of fitting of concave and convex portions (called caulking) forfixation of a laminate causes degradation of a caulked portion and itsneighborhood.

Further, the manufacturing process of mounting a frame on an outerperiphery of a stator core to fix the same to a bracket that supports abearing is in many cases performed in order to prevent a stator frombeing displaced by torque generated between a rotating rotor and thestator, but a force exerted on the outer periphery of the stator core bythe frame has an influence on not only a neighborhood of the outerperiphery of the stator core, through which magnetic flux does not passso much, but also a neighborhood of teeth, which makes a main passage,to cause degradation of flat rolled magnetic steel sheets and strip,which makes a main passage of magnetic flux, in magnetic property anddisplacement of teeth, thus changing an inside diameter shape of thestator core.

Unless gaps and magnetic property are ideally uniformly formed for thenumber of poles and the number of slots, cogging torque of lower ordersis generated.

As described above, cogging torque of orders consistent with the numberZ of slots is generated due to non-uniformity on a side of a magnet, andcogging torque of orders consistent with the number 2 p of poles isgenerated due to various factors, such as non-uniformity in air gap,non-uniformity in open width, non-uniformity in clearance gap,distribution of magnetic property with respect to magnetic anisotropy offlat rolled magnetic steel sheets and strip, distribution of magneticproperty generated by partial degradation of magnetic permeability dueto punching, caulking, and stress in a frame, nonuniform distribution ofclearance gaps of a split core, structural non-uniformity of joints,etc.

These factors are inevitably generated in actual motors by amanufacturing method for enhancement in volume production, or a limit inworking accuracy in manufacturing processes.

Trials for reduction in cogging torque have been made taking notice ofsuch manufacturing processes. For example, in order to obtain uniformityin air gaps, JP-A-2001-218429 (see Patent Document 1) proposes measuresof ensuring roundness of inside diameter by uniformly applying pressureover outer and inner peripheries of a core to fix the same when a statoris to be press fitted into a frame. In JP-A-09-23687 (see PatentDocument 2), it is tried to reduce cogging torque due to magneticanisotropy by displacing a direction of magnetic anisotropy from centralangles of teeth.

Also, in JP-A-2001-95199 (see Patent Document 3), it is tried to preventan increase in cogging torque by maintaining a frame as uniform aspossible in thickness to maintain a force given to a stator by the frameuniformly and to prevent the stator from being nonuniformly changed ininside diameter shape. In JP-A-2001-258225 (see Patent Document 4) andJP-A-2002-272074 (see Patent Document 5), measures for restriction onthe number of caulked portions have been proposed taking account ofinfluences by caulking. Further, in JP-A-06-52346 (see Patent Document6), lamination is made so as to arrange seams circumferentially atsubstantially equal intervals whereby it is tried to dissolvenon-uniformity in magnetic flux, which is caused by the seams.

The inventors of the present application have researched componentshaving the same orders as the number 2 p of poles, among cogging torquehaving a smaller number of pulsation than a least common multiple of thenumber 2 p of poles of a magnet and the number Z of slots of a statorand clarified that products manufactured in volume production appear inmany cases as a result of superposition of cogging torque waveformsincluding an amplitude and a phase, of at least two or more factors.Accordingly, a fundamental understanding is obtained, in which measurestaking notice of only one property as in the prior art are insufficientto enable adequately reducing cogging torque and measures for individualproperties, for example, trying to make roundness approach zero cannotmaterialize a complete ideal state actually. In particular, for actualmotors manufactured in volume production, it is difficult to unlimitedlymake cogging torque approach zero without taking account of workingaccuracy. That is, a technique is demanded, in which cogging torquecaused by working accuracy is grasped as net and cogging torque measuredin a final stage of the manufacturing process is cancelled to be madezero.

Non-Patent Document 1: Materials of workshop of rotating machinery ofElectric Appliance Society RM-03-152 (2003)

Patent Document 1: JP-A-2001-218429

Patent Document 2: JP-A-09-23687

Patent Document 3: JP-A-2001-95199

Patent Document 4: JP-A-2001-258225

Patent Document 5: JP-A-2002-272074

Patent Document 6: JP-A-06-52346

In permanent-magnet type synchronous motors, cogging torque having thesame orders as the number 2 p of poles of a magnet is generated due tocomposite superposition of various factors, such as non-uniformity inair gap, non-uniformity in open width, non-uniformity in clearance gap,distribution of magnetic property with respect to magnetic anisotropy offlat rolled magnetic steel sheets and strip, distribution of magneticproperty generated by partial degradation of magnetic permeability dueto punching, caulking, and stress in a frame, nonuniform distribution ofclearance gaps of a split core, structural non-uniformity of joints,etc. In this case, it is necessary to take notice of superposition ofcogging torque waveforms including not only amplitudes but also phases,and it is necessary to take canceling measures to make cogging torque,which appears as a result of superposition, approach zero asconsequence, in addition to taking compensating measures to separate andcorrectly estimate respective factors, in which amplitudes negate otherproperties and is apparently decreased, and to reduce individualamplitudes.

The invention has been thought of in order to solve the problemsdescribed above, and has its object to provide a permanent-magnet typesynchronous motor, in which cogging torque having pulsating componentsof the same orders as the number 2 p of poles of a magnet is decreasednear to zero unlimitedly by separating composite individual factors ofcogging torque and taking thorough measures of reducing the cause oflarge amplitudes in an experimental manufacturing stage, and regulatingprocesses to superpose properties, in which phase control is possible,on properties, in which amplitudes cannot be decreased for theconvenience of manufacture, to cancel the latter, and a method ofmanufacturing the same.

SUMMARY OF THE INVENTION

The invention comprises a stator having Z (Z is a natural number) slots,on which a coil is arranged, a rotor having permanent magnets of 2 p (pis a natural number) poles and inserted into a torus of the stator, anda frame that pressurizes an outer periphery of the stator inward in Nlocations (N is a natural number) with larger forces than those forother portions. Specifically, in case of shrinkage fit and a moldingprocess, a frame, an external form of which is not a torus but has issubstantially square to have a thickness distribution, may be made useof, and in the case where the frame is substantially a torus in shape,pressurizing parts such as spacer, etc. are partially added to makepressurized regions N in number. In the case where the frame is notsubstantially circular in outer shape and pressurized points are smallerin number than N, pressurizing parts are added to increase pressurizedpoints in number. In case of using pressurization or pressurizing parts,a mechanism is made capable of adjusting pressurized regions and adegree of pressurization.

In processes until mounting of the frame, marking affordingdiscrimination of individual teeth or slots of the stator is made in oneor more locations and made a reference position. In the case where asplit core is adopted to include seams as joined, the seams can be madea reference position. In a stage of trial manufacture, with respect tocogging torque having components of the same orders as the number 2 p ofpoles and caused by the stator, a state of generation is separated bycharacteristics and grasped in a manufacturing process before mountingof the frame by measuring cogging torque of a stator without a frame, ormeasuring cogging torque of a stator, on which a torus having a highaccuracy in shape and subjected to influences of stress by the frame assmall as possible is experimentally mounted.

In a process of mounting the frame on the stator, a feature resides inthat fixation is made after the positional relationship between thereference position of the stator and pressurizing regions in N locationson the frame is determined in terms of N number and an angle, whichcancel the state of cogging torque before mounting of the frame. Anglesbetween the reference position and pressurized points are determined onthe basis of data, in which cogging torque generated in a manufacturingprocess before mounting of the frame and the cause for generation of thecogging torque are separated by characteristics.

According to the invention, it is possible to obtain a permanent-magnettype synchronous motor, in which cogging torque attributable tonon-uniformity of a stator is cancelled to decrease an entire coggingtorque, by giving to predetermined locations on the statornon-uniformity of magnetic property caused by stress, air gap on aninside diameter caused by stress, open width, and displacement ofclearance gap.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a cross section of a permanent-magnet typesynchronous motor perpendicular to an axial direction of a stator,according to an embodiment 1 of the invention.

FIG. 2 is a view showing a cross section of a permanent-magnet typesynchronous motor perpendicular to an axial direction of a stator,according to an embodiment 2 of the invention.

FIG. 3 is a view showing a cross section of a permanent-magnet typesynchronous motor perpendicular to an axial direction of a stator,according to an embodiment 3 of the invention.

FIG. 4 is a view showing a cross section of a permanent-magnet typesynchronous motor perpendicular to an axial direction of a stator,according to an embodiment 4 of the invention.

FIG. 5 shows a thickness distribution of a frame, in a normal direction,in the embodiment 4 of the invention.

FIG. 6 is a view illustrating an articulation type iron core in theembodiment 5 of the invention.

FIG. 7 is a view illustrating butt portions of the articulation typeiron core in the embodiment 5 of the invention.

FIG. 8 shows results of actual measurement of lower-order components ofcogging torque in the case where positioning is made in the buttportions of the articulation type iron core in the embodiment 5 of theinvention and manufacture is carried out.

FIG. 9 is a view showing a cross section of a permanent-magnet typesynchronous motor perpendicular to an axial direction of a stator,according to an embodiment 6 of the invention.

FIG. 10 shows an example of results of measurement of cogging torque atan angle of a rotor in the permanent-magnet type synchronous motoraccording to the embodiment 6 of the invention.

FIG. 11 is a view showing a cross section perpendicular to an axialdirection of a permanent-magnet type synchronous motor according to anembodiment 7 of the invention.

FIG. 12 shows an example of results of measurement of cogging torque atan angle of a rotor in the permanent-magnet type synchronous motoraccording to the embodiment 7 of the invention.

FIG. 13 is a view showing a cross section perpendicular to an axialdirection of a permanent-magnet type synchronous motor according to anembodiment 8 of the invention.

FIG. 14 is a transverse, cross sectional view showing a permanent-magnettype synchronous motor according to the embodiment 8 of the invention.

FIG. 15 is a view showing a cross section perpendicular to an axialdirection of a permanent-magnet type synchronous motor according to anembodiment 10 of the invention.

EMBODIMENT 1

FIG. 1 is a view illustrating a method of assembling a motor in theembodiment 1 to carry out the invention. In FIG. 1, a stator iron core(stator core) 1 is constituted by laminating flat rolled magnetic steelsheets and strip, in which twelve teeth 2 and twelve slots 3 are formed.A frame 4 (referred below to as circular frame) is predetermined to becircular as a cross sectional shape of a frame 4 perpendicular to arotor rotating shaft (not shown) in this example. Screw holes 5 forassembly of a rotor and a stator are provided to be opposed to eachother at an angle of 180 degrees. Since the frame 4 is machinedconforming a die, it is substantially equivalent in form accuracy to thedie, and a shape of that hole of the frame 4, which accommodates thereinthe stator iron core 1, is not so high in roundness (a difference ofmaximum/minimum dimensions of an inside diameter), the shape being anelliptical shape of, for example, about 120 μm for products of a certainseries.

FIG. 1 shows a straight line 6 indicative of a minor axis of theelliptical shape. Also, an outside diameter shape of the frame 4 issubstantially similar to an inside diameter shape, and has an ellipticalshape. Accordingly, the frame 4 is substantially constant in thicknessin a circumferential direction and it is found that as a result of shapemeasurement that the screw holes 5 for mount are surely located in adirection of major axis of the ellipse. Accordingly, the thickness ofthe frame on the major axis is decreased by a magnitude amounting to adiameter of the screw holes.

On the other hand, the stator iron core 1 is generally fabricated bylaminating cut portions of flat rolled magnetic steel sheets and stripwhile caulking them, and an outside diameter of the stator iron core 1has a roundness of, for example, 50 μm or less and is in many cases saidto be substantially circular as compared with the shape of the hole ofthe frame 4.

Conventionally, when the stator iron core 1 is to be inserted into andfixed to the hole of the frame 4, the positional relationship of theframe 4 and the stator iron core 1 in a direction of rotation about arotor rotational axis is not taken account of, but the stator iron core1 is fixed to the frame 4 in an optional position by means of methodssuch as shrinkage fit, press fit, molding, etc.

Paying attention to the fact that the hole of the frame 4 is in manycases inferior in roundness to an external form of the stator iron core1 as described above, the invention takes notice of usefulness incontrolling the positional relationship in the direction of rotation ina fixing process, in which the frame 4 and the stator iron core 1 arefixed together in the manufacturing process.

For example, in the case where a stator iron core is fixed to a frame bymeans of shrinkage fit, the stator iron core in a normal temperaturestate is conformed to the frame, which is expanded in shape, and left atnormal temperature, and in the meantime the frame is contracted in shapeto clamp the stator iron core, at which stress is applied to amaximum-diameter side of the stator iron core by the frame. That is, forthe frame 4, of which a hole has an elliptical-shaped cross section, andthe stator iron core 1 having a substantially circular-shaped crosssection, the frame 4 and the stator iron core 1 contact with each otherin a direction along the minor axis of the elliptical shape in aninitial stage of the fixing process.

Further, it can be generally said that the larger the thickness of theframe, the larger the shape deformation at the time of expansion andshrinkage, and the larger the thickness of the frame, the larger a forceapplied directly on an outermost diameter portion of a side of thestator iron core by the frame. Accordingly, in fixation in such case, alocation, in which stress of the stator iron core 1 becomes extreme(maximum in this case), corresponds to a location, in which the straightline 6 consistent with the minor axis intersects an outside diameter ofthe stator iron core 1. In the embodiment, the location, in which stressof the stator iron core 1 becomes extreme, is made nearest to the teeth2 of the stator iron core 1. Here, since the teeth 2 is even in number,in order to realize the arrangement, it suffices that the straight line6 corresponding with the minor axis be caused to be consistent with ateeth center line 7, which connects between the teeth 2 positioned atopposite poles of the stator iron core 1, or made nearest thereto.

While teeth center lines 7 corresponding in number to a half of thetotal number of teeth, that is, a plurality of teeth center lines can beset, the teeth center line 7 being positionally registered with thestraight line 6 may be any one of the plurality of teeth center lines 7.After positioning in this manner, the both elements are fixed while themutual positional relationship is maintained. Taking as an examplefixation with shrinkage fit, the hole of the frame 4 and the externalform of the stator iron core 1 are first measured along with the shapesthereof at an environmental temperature T0. Normally, measurement everyframe and every stator iron core is not necessary provided that typicalsamples are measured.

Subsequently, the frame 4 is raised in temperature up to a specifictemperature T1. T1 can be beforehand found through computationalestimation as a temperature until the hole of the frame 4 is increasedin diameter by thermal expansion to afford insertion of the stator ironcore 1 into the hole provided that a material and a shape for the statoriron core 1 are given.

In this manner, the stator iron core 1 is inserted into the frame 4,which has been heated to temperature T1. Then the stator iron core 1 isrotated or the like to be positioned relative to the frame 4 so that thestraight line 6 corresponding to the minor axis of the hole of the frame4, which has been found, is made consistent with the teeth center line 7of the stator iron core 1. After positioning in this manner, the frame 4is cooled to normal temperature T0 to allow the stator iron core 1 to befixed to the frame 4 through shrinkage at the time of cooling.

As described above, the straight line 6 corresponding to the minor axisof the frame 4 and the teeth center line 7 of the stator iron core 1 arepositioned and fixed together whereby cogging torque is decreased ascompared with the case where fixation is made in other locations. It isthought that the cause for this is that since the stator iron core 1 islarger in thickness by the teeth 2 than that in other locations to belarge in mechanical strength, a region having an influence on a passage,through which magnetic flux passes inside the stator iron core 1, issmall and hard to be influenced by stress when magnetic flux passes.

With a permanent-magnet type motor, assuming that a rotor and a stator,which include a magnet, take theoretical values, those times, in whichcogging torque pulsates when a rotor makes a round, become thosecorresponding to a least common multiple of the number of poles of amagnet and the number of slots of a stator as shown on pages 2 to 4 ofthe Non-Patent Document 1. In actual products, however, there isoccurred the number of pulsation smaller than a least common multiple ofthe number of poles of a magnet and the number of slots of a stator,which is typical in pulsating components being the same in number as thenumber of slots of a stator, and integral times thereof, or as thenumber of poles of a magnet and integral times thereof.

Among these, the formula (19) on page 4 of the Non-Patent Document 1shows that one of occurrence conditions for pulsating components ofcogging torque, which are the same in number as the number of poles of amagnet, is the case where the permeance distribution function formed bya stator has those pulsating components of N times every revolution of arotor, which has a predetermined condition, and further shows the Npredetermined conditions. That is, pulsating components of permeanceformed by a stator side constitute one of causes for generation ofpulsating components of cogging torque, which are the same in number asthe number of poles of a magnet. In case of 8 poles and 12 slots, N=4 isin the stator iron core and N=4 as higher-order components created byN=2 also makes a cause for lower-order pulsating components.

Mechanism for occurrence of influences on cogging torque as a result ofapplication of a force on a stator iron core from a frame at the time offixation of the frame are roughly classified into two. One of them is aproblem with deformation of an iron core. That is, when a forcepropagates inside the iron core, it is problematic how positions ofteeth tip ends and open widths (these have an influence on air gaps forpassage of magnetic flux) are varied as compared with those before beingfixed to the frame and what nonuniform distribution they have as seenevery tooth when the frame and the iron core balance in a final statewith respect to stiffness (hardness or the like).

The second one of them is that energy, which cannot be finally deformedand is left as residual stress inside the iron core, partially changesthe magnetic property (easiness, with which magnetic flux passes,magnetic permeability) of the iron core, and consequently how a manner,in which magnetic flux passes, is changed and what nonuniformdistribution is generated.

For these changes, structural analysis enables computing of a state offinal deformation and a state, in which residual stress is distributed.In particular, the distribution of residual stress makes a cue to knowhow a passage, through which magnetic flux passes, is varied, and inwhat region such change comes out, as seen every tooth.

As an example, structural analysis was made in the case where acompletely circular stator iron core was shrinkage fitted into a frame,which is circular in inside diameter and elliptical in outside diameter,with the result that residual stress became larger in amplitude ofdouble-symmetrical pulsation with the case where a maximum point ofstress was made consistent with a slot center, than with the case wherea maximum point of stress was made consistent with a teeth center. It isthought that since residual stress changes the magnetic property of flatrolled magnetic steel sheets and strip, the magnetic property of flatrolled magnetic steel sheets and strip also becomes in amplitude ofdouble-symmetrical pulsation and cogging torque having lower-orderpulsating components is increased. That is, in order to decrease coggingtorque having lower-order pulsating components, it is thought thatmaking a maximum point of stress consistent with a teeth center is moreeffective than making a maximum point of stress consistent with a slotcenter.

However, instead of positioning the maximum point of stress nearest tothe teeth 2, it is also in some cases effective to position the pointnearest to a center of the slot 3. That is, a slot center line 8connecting between centers of slots 3 positioned at opposite poles ismade consistent with the straight line 6. The effect of decreasingcogging torque is recognized in this case except the case wherepositioning is made on the basis of the teeth center line 7. Since inthis position, the stator iron core 1 is smaller in thickness than inother locations but an improvement in cogging torque is recognized, thatdirection, in which stress is applied, is not radial on thecircular-shaped cross section of the stator iron core 1 but angularrelative to the radial direction, and residual stress itself isdispersed to become small in amplitude, which appears to be effective indecreasing cogging torque, as shown in the structural analysis.

This is because cogging torque finally appears not only as a change inmagnetic property caused by the residual stress but also as a result ofa combination of this change with flows of magnetic flux when an actualmotor operation is performed, and it is thought that with which of ateeth center and a slot center the maximum point of stress should beoptimally made consistent is changed according to conditions such as adetailed shape of a stator iron core, direction of magnetization of apermanent magnet on a rotor side, magnitude of magnetic flux, etc., sothat it is desirable to make use of structural analysis to determine anoptimum position as shown in an embodiment 2.

Also, since it is thought that also in the case where a combination ofthe number of poles and the number of slots is different from that inthe present embodiment, pulsating components appearing in cogging torquechange depending upon whether stress applied on a stator iron core froma frame is applied on a side of a slot of the stator iron core or a sideof teeth, it is necessary to make constant a manner, in which stress isapplied on the stator iron core from the frame, in order to suppressdispersion in cogging torque. Therefore, it is necessary to take noticeof a point, at which stress applied on the stator iron core from theframe is maximum or minimum, to position a teeth center and a slotcenter of the stator iron core relative to the point.

In this manner, cogging torque in the both cases is decreased ascompared with the case where fixation is made in other positions, andfurther provided that the frame 4 and the stator iron core 1 are fixedtogether in a controlled state, in which the constant positionalrelationship is maintained in this manner, cogging torque is madeuniform in magnitude as far as the same type of device is associated.Since a method of controlling such mutual fixed positional relationshipconstant is not conventionally adopted, cogging torque in conventionalproducts involves a large dispersion in magnitude and an increase in anaverage statistical center value of the cogging torque and products arereduced in yield in the case where magnitude of cogging torque is madean index of product control. According to the invention, dispersion inmagnitude of cogging torque is improved and products are enhanced inyield conjointly with the effect of reduction in cogging torque.

Such effect is obtained only by simple positioning and fixation and doesnot need that complex manufacturing process, in which fins disclosed inPatent Document 3 is manufactured, so that it can simplify themanufacturing process to be also effective in reduction in cost. Also,according to the invention disclosed in Patent Document 3, a frame ismuch reduced in effective thickness and involves a fear in mechanicalstrength, but the present invention involves less reduction in effectivethickness and is excellent in this respect.

EMBODIMENT 2

FIG. 2 shows a method of assembling a motor in the embodiment 2 of theinvention, and the same circular frame as that in the embodiment 1 istaken as an example in the figure. The reference numerals are the sameas those in FIG. 1.

In the embodiment 2, there is illustrated the case where a position of amaximum point of stress applied on a stator iron core 1 from a frame 4is not clear from shapes thereof. Stated taking, for example, shrinkagefit as an example, a hole of the frame 4 and an external form of thestator iron core 1 are both elliptical, and it is not clear whichlocation of the stator iron core 1 comes first into contact with theframe in the case where the frame 4 is cooled. Accordingly, the maximumpoint of stress is not clear.

In order to determine a position of a maximum point of stress in suchcase, it suffices to make use of, for example, a structural analysisprogram to determine a distribution of stress applied on the stator ironcore 1 from the frame 4.

When shape and material of the frame 4, shape and material of the statoriron core 1, conditions of mutual arrangement, and conditions oftemperature are input, the structural analysis program can be used tocalculate a distribution of stress applied on the stator iron core incase of fixation through, for example, shrinkage fit. A predeterminedarrangement can be determined by finding an arrangement, in which amaximum value of stress comes to a position corresponding a teeth centerline 7 or a slot center line 8 on the basis of results of thecalculation with respect to conditions (specifically, for example, anangle of rotation is changed) of plural arrangements of the frame 4 andthe stator iron core 1.

In addition, since stress is applied on a side of the stator iron corefrom the frame to propagate inside while being accompanied by strainwith the final result that distribution and direction of residual stressare found in this method, it is in some cases preferred from theviewpoint of reduction in cogging torque that a maximum value of stresspreferably come to other position on the basis of such informationinstead of having the maximum value coming to a neighborhood of teeth,or a neighborhood of a slot. This leads to judgment on the basis of adirection of stress and a thickness of the stator iron core 1 in thedirection.

In addition, when positioning is made in a position once determined forthe type of device to fix the frame 4 and the stator iron core 1together, there is produced an effect of reduction in dispersion inaddition to the effect of reduction in cogging torque as described inthe embodiment 1. Accordingly, as compared with a conventional method ofmanufacturing a motor, in which fixation is made without positioning ina fixed positional relationship, an improvement in dispersion ofmagnitude of cogging torque and the effect of reduction in coggingtorque are combined to enhance products in yield.

The above matter is likewise established even when an external form ofthe frame 4 is rectangular or otherwise, and the same effect can beobtained.

EMBODIMENT 3

FIG. 3 is a view illustrating a method of assembling a motor in theembodiment 3 of the invention. The same reference numerals as those inFIG. 1 denote the same parts. A frame 4 is a square one, a hole of theframe 4 is substantially circular in cross section perpendicular to arotating shaft of a rotor, and an external form of a stator iron core 1is also substantially circular.

In such case, since the thickness of the frame 4 has a cleardistribution in a circumferential direction, stress applied on a side ofthe stator iron core 1 from the frame 4 is dependent upon the thicknessof the frame 4, and it is thought that the larger the thickness of theframe in a normal direction, the larger the stress applied on the side.That is, stress applied on the stator iron core 1 from the frame 4 isincreased in a diagonal direction, in which the thickness of the frame 4is increased as compared with the other case.

Accordingly, the effect of reduction in cogging torque like the effectdescribed in the embodiment 1 can be obtained by making positioning soas to make a teeth center line 7 of the stator iron core 1 consistentwith either of two diagonal lines 9 of the frame 4 in order to set theteeth center line to a maximum point of stress, and fixing the statoriron core 1 to the frame 4. Also, in the case where the thickness of theframe has a clear distribution in the normal direction and stressdependent upon the thickness is applied to the stator iron core and inthe case where the distribution is tetra-symmetrical relative to 360degrees of a machine angle, the number of slots is 12, and the number ofslots is divided by 4 being a symmetric property to result in 3 beingodd, it is not necessarily necessary to take notice of only a point, atwhich stress becomes maximum, and even when taking notice of a point, atwhich stress becomes minimum, a slot center line of the stator iron coreis made consistent with a side center line of the frame being a point,at which stress becomes minimum, it results that a teeth center is madeconsistent with a point, at which stress becomes maximum.

In this manner, it can be said that according to a shape of a frame andthe number of slots it is unnecessary to take notice of only a point, atwhich stress becomes maximum, and it does not matter if a reference forpositioning is determined taking notice of a point, at which stressbecomes minimum. Details of the manufacturing method are the same asdescribed in the embodiment 1 except the method of positioning.

In addition, while fixation of a frame and a stator iron core is made bymeans of shrinkage fit in the embodiments 1 to 3, such fixation may bemade by means of press fit, or an adhesive, and the way of fixation isnot particularly limitative.

Further, while the invention exemplifies a stator having 12 slots, thecase with other pole slots will do, and there is no limitation thereon.

Besides, an external form of a frame having a circular-shaped or asquare-shaped cross section is exemplified in the respectiveembodiments, a triangular or a pentagonal cross section will do, andthere is no limitation in a shape of a frame.

EMBODIMENT 4

FIG. 4 is a view illustrating a method of assembling a motor in theembodiment 4 of the invention. The same reference numerals as those inFIG. 1 denote the same parts. A frame 4 is a substantially square one,and includes substantially circular notches 4 a in diagonal directions.Also, although being not shown, a connector box or the like is in somecases mounted above the frame to have an influence on a thickness of theframe 4.

In such case, since a thickness distribution of the frame 4 cannot bereadily known, the angle dependence of frame thickness in a normaldirection is found as shown in FIG. 5.

As a result, it is found that a region, which is thick in thicknessdistribution and in which stress applied on the stator iron core 1 fromthe frame 4 becomes maximum, is steep and a region, which is minimum inthickness distribution and in which stress applied on the stator ironcore becomes minimum, is gentle in change.

As described above, since with the tetra-symmetrical frame, the numberof slots is 12 and the number of slots is divided by 4 being a symmetricproperty to result in 3 being odd, the equivalent arrangement resultseven when positioning is made taking notice of a point, at which stressbecomes minimum.

In particular, in the case where a point, at which stress becomesmaximum, has a steep rate of change as in the frame configuration of theembodiment, a finite positioning accuracy is inevitably existent in viewof the manufacturing process, so that positioning in that arrangement,in which the rate of change becomes minimum, is hard to be affected byan error in positioning at the time of volume production.

Hereupon, according to the embodiment, the stator iron core 1 and theframe 4 are positioned in a range of a positioning accuracy, formass-produced motors, ±10 degrees centering on an angle, at which stressbecomes minimum, and the frame 4 is fixed. By doing this, the effect ofreduction in cogging torque like the effect described in the embodiment1 can be obtained, in the case where the positioning accuracy is finitein view of the working process, positioning can be made in a state ofbeing hard to be affected by the error, and it is possible to maintaincogging torque at a small value in volume production without dispersion.

EMBODIMENT 5

In the embodiment, the same frame as that in the embodiment 4 is usedbut an articulation type iron core is adopted for a stator iron core.This is contrived so that instead of punching a substantially circularseamless iron core from flat rolled magnetic steel sheets and strip, oneor more cuts are provided and bendable hinge mechanisms (articulations10) are provided on slots as shown in FIG. 6 to make a stator iron corestraight to easily perform winding in a process, in which winding iswound round teeth.

Since the stator iron core includes cuts, it is necessary to put thecuts together after winding to perform the connection work, such aswelding, etc., on a side of the iron core. These portions are calledbutt portions. FIG. 7 is a schematic view showing the stator iron coreto clarify the butt portions 11. The butt portions 11 of the iron corehave different properties in structure and internal state from otherslot portions such that residual stress at the time of welding is left.Therefore, in case of that articulation type iron core, in which thebutt portions 11 are present, it is thought desirable that stressapplied on the butt portions from the frame be small.

Hereupon, according to the invention, instead of a slot center line, acenter line of the butt portion 11 is made a reference point ofpositioning, and a target point of positioning is set so that the buttportion 11 is conformed to an arrangement, in which stress applied onthe stator iron core from the frame becomes surely minimum, that is, inwhich a thickness becomes minimum and its rate of change also becomesminimum in the thickness distribution in a normal direction of theframe. The positioning accuracy is equivalent to that in the embodiment4 to be ±10 degrees.

FIG. 8 shows a summary of results of actual measurement of lower-ordercomponents of cogging torque in the case where positioning is made asdescribed above and manufacture is carried out. Even when an positioningangle is dispersed in a range of ±10 degrees, the components areencompassed in 0.05[arb.unit] or less, which is aimed at. In addition, asolid square indicates data in the case where a positioning angle isintentionally dispersed at 22 degrees. It is seen from the results ofactual measurement that cogging torque is surely decreased in case ofpositioning as compared with the case where positioning is not made.

In this manner, even in the case where an articulation type iron core,in which the butt portions 11 are present, is used, the effect ofreduction in cogging torque like the effect described in the embodiment1 can be obtained by making positioning making the butt portion of thestator iron core consistent with a target point, in which stress appliedon the stator iron core from the frame, that is, the thicknessdistribution in a normal direction of the frame becomes minimum and therate of change of the thickness distribution becomes minimum, and in thecase where the positioning accuracy is finite in view of the workingprocess, positioning can be made in a state of being hard to be affectedby the error, so that it is possible to maintain cogging torque at asmall value in volume production without dispersion.

Also, it goes without saying that the present method is not limited toan articulation type iron core but can be used for a thin-wallconnection type iron core of that type, in which connections of teethare made from a thin wall.

EMBODIMENT 6

The embodiment 6 will be described with respect to a permanent-magnettype synchronous motor having 8 poles and 12 slots and capable ofdecreasing cogging torque by applying pressure in predeterminedpositions with a pressure part. FIG. 9 is a cross sectional view showinga permanent-magnet type synchronous motor according to the embodiment 6.In the embodiment, an explanation will be given to the case of 8 polesand 12 slots.

A rotor 12 comprises a shaft 12 a, a rotor yoke 12 b, and magnets 12 cbeing permanent magnets. The shaft 12 a secures thereto the rotor yoke12 b, which comprises a magnetic body being octagonal in external form,and the magnets 12 c are fixed to respective flat portions of theoctagonal external form of the rotor yoke 12 b. Adjacent poles of themagnets 12 c are configured to be opposite to each other. A stator ironcore (stator core) 13 is mainly composed of teeth 13 a and a back yoke13 b on a circular pipe, and an inner-ring side of the teeth 13 a of thestator 13 and arcuate sides of the magnets 12 c of the rotor 12 arearranged to define an air gap. In addition, a coil normally wound roundthe teeth 13 a is omitted in FIG. 1.

Also, provided outside the stator 13 are pressurizing parts 14 thatpressurize the stator 13 inward in predetermined positions on an outerperiphery thereof, and an armour part 15 that pressurizes thepressurizing parts 14 inward at outer peripheries of the pressurizingparts 14. Also, although being not shown, the rotor 12 is rotatablysupported, and the magnets 12 c in the embodiment are ideally arrangedto provide for a uniform and symmetrical magnetic flux densitydistribution. On the other hand, since the stator 13 includes nonuniformportions in terms of manufacture, pulsating components are contained inpermeance. An influence, which is produced on a product by the pulsatingcomponents, will be described later.

Subsequently, an operation will be described. The rotor 12 and thestator 13 are assembled together, the rotor 12 is rotated at low andconstant speed in a state, in which electric current is not caused toflow through a coil (not shown) around the teeth 13 a, and torquerequired for rotation at that time is measured every angle. Such torqueis called loss torque. The loss torque is composed of a specific portiontypified by slide torque of bearings that rotatably support the rotor12, and pulsating components according to a variation of the rotor 12every angle in total magnetic energy of that magnetic circuit, which isformed by the rotor 12 and the stator 13. In addition, the pulsatingcomponents are called cogging torque.

FIG. 10 shows an example of results of measurement of cogging torque atan angle of the rotor in a permanent-magnet type synchronous motoraccording to the embodiment 6. The angle of the rotor is shown to rangefrom 0 to 90 degrees. In the figure, the axis of abscissas indicates arotor angle and the axis of ordinate indicates cogging torque, a curve abeing measured before the pressurizing parts 14 pressurize the stator 13in predetermined positions on an outer periphery thereof at the time ofmanufacture. A reference of the rotor angle assumes 0 degree in the casewhere the angularly positional relationship of the rotor 12 and thestator 13 is put in a state of arrangement shown in FIG. 9. That is, inthe case where the reference angle of the rotor 12 is intermediatebetween magnetic poles, a position of the reference angle of the rotor12 and a center of the teeth 13 a of the stator 13 are disposed on astraight line.

The curve a in FIG. 10 contains components obtained by superposition ofpulsating components of 24 times and pulsating components of 8 timeswhen the rotor 12 makes one revolution. Pulsating components of 24 timesare known as pulsating components that can be generated even when thestator 14 and the rotor 12 assume theoretical shapes, since the rotor 12is 8 in number of poles, the stator 13 is 12 in number of slots in theembodiment, and the pulsating components of 24 times is in agreementwith 24, which is a least common multiple of 8 and 12. Pulsatingcomponents of 8 times are the same as the number of poles of the rotor12.

As shown in the formula (19) on page 4 of the Non-Patent Document 1, itis indicated that the rotor 1 can generate cogging torque havingpulsating components of 8 times (2 p) every revolution in the case wherethe permeance distribution function formed by the stator has thosepulsating components of N times every revolution of the rotor 12. It isindicated that N meets any one of the following formulaeN=p  (1)orN=±2 p −i1×Z  (2)orN=i1×Z±2p  (3)where p indicates pole logarithm assuming a value of a half of thenumber of poles, Z indicates the number of slots, and i1 indicatesspatial orders when the permeance distribution function is expanded inFourier series.

Since an order i1 having a largest influence is 1, it does not matter ifi1=1. In the case where it is applied to a permanent-magnet typesynchronous motor having 8 poles and 12 slots in the embodiment 6, aminimum numeral among solutions of N is 4. N=4 is obtained from p=4, andwhen i1=1, Z=12, and p=4, N=1×12-8=4 results. The smaller the solutionof N, the more liable it appears as cogging torque. That is, the factthat cogging torque has pulsating components of 8 times (8 times in aplus direction and 8 times in a minus direction) per one revolution ofthe rotor 12 indicates a possibility that pulsating components of N=4times are contained in the permeance distribution function of the stator13 since the stator 13 is nonuniform in terms of manufacture.

However, the cause for this lists anisotropy, in magnetic permeability,of a steel sheet used for the stator 13, local residual stress generatedby the working at the time of manufacture, stress due to press fittingof the stator 13, etc. as well as the shape of the stator 13, and it isdifficult to specify and remove these causes in terms of manufacture. Inorder to decrease cogging torque having pulsating components of 8 timesper one revolution of the rotor 12, it is considered as effective meansto conversely give components having an opposite phase to that ofpulsating components of 4 times in the permeance distribution functionformed by the stator 13 and to cancel pulsating components of 4 times inthe permeance distribution function.

Here, in the case where pulsating components of 8 times contained incogging torque present a waveform of a phase of a shown in FIG. 10,predetermined stress and displacement are imparted to the stator 13 bypreparing four pressurizing parts 14 corresponding to pulsatingcomponents of 4 times contained in the permeance distribution functionof the stator 13 as shown in FIG. 9, and press fitting the parts in fourlocations at intervals of 90 degrees from a center of teeth disposed ona straight line passing through a position of the reference anglebetween the armour part 15 and the stator 13. The stress changes thestator 13 locally in relative permeability. A change, in relativepermeability, caused by displacement and stress of the stator 13 resultsin pulsating components of 4 times per one revolution of the rotor 12 asan air gap length between the stator 13 and the rotor 12, and relativepermeability also results in pulsating components of 4 times per onerevolution of the rotor 12 due to stress of the back yoke 13 b of thestator 13. Therefore, components having an opposite phase to that ofpulsating components of 4 times in the permeance distribution functionare given to enable canceling pulsating components of 4 times in thepermeance distribution function of the stator 13.

A curve b in FIG. 10 represents cogging torque measured after the stator13 is press fitted into the pressurizing parts 14 with an appropriateinterference. It is found that as compared with the curve a, pulsatingcomponents of 8 times contained in cogging torque per one revolution arecancelled and cogging torque is generally decreased.

Also, in the case where pulsating components of 8 times contained incogging torque are offset a degrees from the phase of the curve a inFIG. 10, it can be accommodated by rotating positions of pressurizationcorrespondingly. While a quantity, by which positions of pressurizationare to be made offset, generally tends to be in proportion to a degrees,proportioning does not occur occasionally since the tendency isdependent upon the degree of interference in case of press fit.Accordingly, it is desirable to experimentarily grasp the quantityaccording to conditions of individual products.

The number of locations of pressurization is determined by a plus valueof a solution of N found by N=p, N=±2×p−Z, or N=Z±2×p where i1=1 in theformulae (1) to (3), and positions of pressurization are determined suchthat a point, at which cogging torque is zero in measurement of coggingtorque, or a neighborhood thereof makes a first position ofpressurization, the first position of pressurization is made a referenceangle in FIG. 9, and the remaining positions of pressurization arearranged at equal angular intervals.

As described above, since the embodiment comprises the stator 13 having12 slots, on which a coil is arranged, the rotor 12 having permanentmagnets of 8 poles and inserted into a torus of the stator 13, and thepressurizing parts 14 that pressurize an outer periphery of the stator13 inward in N locations, N being a plus minimum value of 4 calculatedfrom N=4, N=±2×4−12, or N=12±2×4, stress and displacement by the stressare imparted to the stator 13 in predetermined locations to cancelpulsating components of 4 times, in permeance, formed by the stator 13,thereby enabling decreasing cogging torque having pulsating componentsof the same number as the number of poles of the rotor 12.

In addition, while the embodiment adopts a construction, in which thearcuate-shaped pressurizing parts 14 are arranged between the stator 13and the armour part 15, a construction, in which stress can be appliedto an inside of an outer periphery of the stator 13 in fourpredetermined locations, suffices, so that in case of application to,for example, products of volume production, stress can be applied insidein predetermined locations by contriving a molding direction and a shapeof a mold as by increasing an outside thickness at the time of moldingan armour in the case where predetermined locations of stressapplication are substantially the same every product.

Also, according to the embodiment, a ratio of the number of poles of therotor 12 to the number of slots of the stator 13 is 2:3, and typicalexamples of the ratio of the same number as that include 4 poles and 6slots, and 6 poles and 9 slots. A minimum among those solutions of Nassociated with pulsating components of N times in the permeancedistribution function of the stator 13, which possibly generatepulsating components, in cogging torque, of the same number as thenumber of poles of the rotor, is N=2 in case of 4 poles and 6 slots andN=3 in case of 6 poles and 9 slots. These produce the same effectprovided that the pressurizing parts 14 of corresponding numbers,respectively, are arranged since slots of the stator 13 and poles of therotor 12 are quite the same in angular relationship when an angle of therotor 12 is regarded as electrical angle. In addition, while accordingto the embodiment N assumes a plus minimum value among values found byN=p, N=±2×p−Z, or N=Z±2×p, N is not limited to the minimum value but mayassume any one of plus values.

EMBODIMENT 7

The embodiment 7 will be described with respect to a permanent-magnettype synchronous motor having 10 poles and 12 slots and capable ofdecreasing cogging torque by applying pressure in predeterminedpositions with a pressurizing part.

FIG. 11 is a cross sectional view showing a permanent-magnet typesynchronous motor according to the embodiment 7. In contrast to theembodiment 6, a rotor has 10 poles and a stator has 12 slots in thepresent embodiment. However, while the pressurizing parts 14 arearranged in four locations in the embodiment 6, they are arranged onlyin two locations in the present embodiment.

Also, magnets 12 c in the embodiment are ideally arranged to provide fora uniform and symmetrical magnetic flux density distribution on theother hand, since the stator 13 includes nonuniform portions in terms ofmanufacture, pulsating components are contained in permeance. Aninfluence, which is produced on a product by the pulsating components,will be described later.

FIG. 12 shows an example of results of measurement of cogging torque atan angle of the rotor 12 in a permanent-magnet type synchronous motoraccording to the embodiment 7. The angle of the rotor is shown to rangefrom 0 to 360 degrees. FIG. 12( a) shows measurement before thepressurizing parts 3 pressurize the stator 13 in predetermined positionson an outer periphery thereof at the time of manufacture.

Cogging torque in FIG. 12( a) contains components obtained bysuperposition of pulsating components of 12 times equal to the number ofslots of the stator 13 and pulsating components of 10 times equal to thenumber of poles of the rotor 12 when the rotor 12 makes one revolution.With respect to pulsating components of 10 times equal to the number ofpoles of the rotor 12, in the same manner as the theory shown in theembodiment 6, conditions of N associated with pulsating components of Ntimes in the permeance distribution function formed by the stator, whichpossibly generate cogging torque having pulsating components of 10 (2 p)times per one revolution of the rotor 12, are any one that meets N=p, orN=±2 p−i1×Z, or N=i1×Z±2 p. Therefore, a minimum numeral among solutionsof N becomes 2 in case of application to a permanent-magnet typesynchronous motor having 10 poles and 12 slots, according to theembodiment 7. It is a case of i1=1, Z=12, and 2 p=10, in whichN=1×12−10.

That is, the fact that cogging torque has pulsating components of 10times (10 times in a plus direction and 10 times in a minus direction)per one revolution of the rotor 12 indicates a possibility thatpulsating components of 2 times are contained in the permeancedistribution function of the stator 13 since the stator 13 is nonuniformin terms of manufacture. As described in the embodiment 6, the cause forthis lists anisotropy, in magnetic permeability, of a steel sheet, localresidual stress generated by the working at the time of manufacture,stress due to press fitting of the stator 13, etc., in addition to theshape of the stator 13.

In particular, for pulsating components of 2 times contained in thepermeance distribution function, after going through the rolling processin manufacture of steel sheet when the steel sheet itself used for thestator 13 is manufactured, residual internal stress is different in adirection of rolling and a direction perpendicular thereto, so that thesteel sheet becomes anisotropic in relative permeability. In the casewhere cores punched from the steel sheet are stacked while being tunedup in direction to form the stator 13, the relative permeability of theback yoke 13 b of the stator 13 will have pulsating components of 2times per one revolution of the rotor 12. In order to decrease pulsatingcomponents of 10 times per one revolution of the rotor 12, it sufficesto conversely give components an opposite phase to that of pulsatingcomponents of 2 times in the permeance distribution function formed bythe stator 13.

Here, in the case where pulsating components of 8 times contained incogging torque present a waveform having a phase as in the curve a inFIG. 2, predetermined stress and displacement are given to the stator 13by preparing two pressurizing parts 3 equal to pulsating components of 2times in the permeance distribution function of the stator 13 as shownin FIG. 11 to press fit the same in an opposite position between thearmour part 15 and the stator 13. The stress changes the stator 13locally in relative permeability. A change, in relative permeability,caused by displacement and stress of the stator 13 results in pulsationsof 2 times per one revolution of the rotor 12 as an air gap lengthbetween the stator 13 and the rotor 12, and relative permeability alsoresults in pulsations of 2 times per one revolution of the rotor 12 dueto stress of the back yoke 13 b of the stator 13. Therefore, when thepositional relationship of the rotor 12 and the stator 13 is put inorder, the permeance distribution function also gives components havingan opposite phase to that of pulsating components of 2 times.

FIG. 12( b) shows cogging torque measured after the pressurizing parts14 are press fitted onto the stator 13 with an appropriate interference.As compared with FIG. 12( a), it is found that pulsating components of10 times contained in cogging torque per one revolution are canceled,and cogging torque is generally decreased.

As described above, since the embodiment comprises the stator 13 having12 slots, on which a coil is arranged, the rotor 12 having permanentmagnets 12 c of 10 poles and inserted into a torus of the stator 13, andthe pressurizing parts 14 that pressurize an outer periphery of thestator 13 inward in N locations, N being a plus minimum value of 2calculated from N=5, N=±2×5−12, or N=12±2×5, stress and displacement bythe stress are imparted to the stator 13 in predetermined locations tocancel pulsating components of 2 times, in permeance, formed by thestator 13, thereby enabling decreasing cogging torque having pulsatingcomponents of the same number as the number of poles of the rotor 12.

EMBODIMENT 8

FIG. 13 is a cross sectional view showing the embodiment 8 of apermanent-magnet type synchronous motor according to the invention. Theembodiment is different from the embodiment 6 in that in place of thepressurizing parts 14 and the armour part 15, a stator 13 is pressfitted into a frame 5. Also, FIG. 14 is a transverse, cross sectionalview showing a permanent-magnet type synchronous motor according to theembodiment 8. The frame 15 comprises pressurizing portions 14 a thatpressurize the stator 13 in four predetermined locations, and bearingportions 14 b that support outer rings of bearings 17 fitted onto arotor 12 so as to support ends of the rotor 12 to enable the rotor torotate. In a constructional view, the bearing portions 14 b and thepressurizing portions 14 a of the frame 15 are provided on the sameaxis.

At the time of manufacture, the stator 13 is press fitted into the frame15 such that the pressurizing portions 14 a of the frame 15 are made inagreement with predetermined locations on an outer periphery of thestator 13. Then the frame 15 pressurizes the stator 13 inward in thepredetermined locations on the outer periphery thereof. Therefore, bygiving the same action as that of the pressurizing portions 14 in theembodiment 6, it is possible to decrease cogging torque. Accordingly,parts can be simplified because of a construction, in which the frame 15can directly pressurize the stator 13 inward in the predeterminedlocations on the outer periphery thereof in addition to enabling theembodiment 8 to produce the same effect as that in the embodiment 6.

In addition, while the embodiment has been described with respect to thecase where press fit is performed as means, by which the frame 15secures and pressurizes the stator 13, shrinkage fit may be performed assuch means.

Also, while the embodiment has been described with respect to the caseof 8 poles and 12 slots, the same effect can be produced provided thatpressurizing portions of the frame 15 are provided in two locations inthe case of 10 poles and 12 slots in the embodiment 7.

EMBODIMENT 9

The embodiment 9 will be described with respect to a manufacturingmethod, in which it is possible to decrease cogging torque in apermanent-magnet type synchronous motor according to the invention.

Generally, in order to stabilize quality at the time of manufacture ofpermanent-magnet type synchronous motors, examination of cogging torqueis commonly made at the time of manufacture. However, the process ofexamination of cogging torque is performed in a stage of finalexamination of finished products after all elements are assembled.Therefore, in the case where a small cogging torque is demandedaccording to specifications of products, those products generated at thetime of manufacture not to meet the specifications are disposed of, ordisassembled for readjustment, or the like. Also, in case ofpermanent-magnet type synchronous motors, which need the process ofpress fit of an external form of a stator 13, the process of press fitrather increases cogging torque in some cases.

The embodiment is described with respect to the case of 8 poles and 12slots in the embodiment 6. First, cogging torque is measured by settingthe rotor 12 and the stator 13 to positions after assembly in a stage,in which the stator 13 is pressurized by the pressurizing portions 14.At this time, it is desirable to perform measurement in a state, inwhich a coil 18 is wound. This is because tension in the coil 18 at thetime of winding generates in some cases stress on teeth of the stator 13or the like.

In addition, in the case where cogging torque at the time of measurementin manufacture is substantially constant in phase every product inproducts of volume production, it is not necessary to performexamination every product but measurement may be made in examinationwithout notice.

Subsequently, the number N of locations, in which the stator 13 ispressurized, is determined from data of cogging torque of the rotor 12every angle. In the case where pulsating components of 8 times being thesame number as the number of poles of the rotor 12 are detected fromdata of cogging torque, there is a possibility that the permeancedistribution function formed by the stator has pulsating components of Ntimes every revolution of the rotor 12, on the basis of the formula (19)on page 4 of the Non-Patent Document 1. It is indicated that N is anyone that meets N=p, or N=±2 p−i1×Z, or N=i1×Z±2 p where p indicates polelogarithm assuming a value of a half of the number of poles, Z indicatesthe number of slots, and i1 indicates spatial orders when the permeancedistribution function is expanded in Fourier series.

A plus minimum numeral among solutions of N becomes 4 as shown in theembodiment 6. Therefore, in order to decrease cogging torque havingpulsating components of 8 times per one revolution of the rotor 12, itis necessary to conversely give components having an opposite phase tothat of pulsating components of 4 times in the permeance distributionfunction formed by the stator 13, so that N is determined to be 4.

Subsequently, those locations, in which the stator 2 is pressurized, isdetermined from data of cogging torque of the rotor 12 every angle.Specifically, components of the same number as the number of poles ofthe rotor 12 are extracted from data of cogging torque, and thoselocations, in which the stator 13 is pressurized, is determinedaccording to a phase of substantially sine wave components thusextracted. For example, in the case where cogging torque is like thecurve a in FIG. 10 in the embodiment 6, the locations of pressurizationare four in number at intervals of 90 degrees from a center of teethdisposed on a straight line passing through a position of the referenceangle shown in FIG. 9.

Also, in the case where pulsating components of 8 times contained incogging torque are offset α degrees from the phase of the curve a inFIG. 10, it is desirable to experimentarily grasp such locations asshown in the embodiment 6, so that it is necessary to beforehand makemapping taking account of conditions such as interference, etc. to takeout information from the map at the time of production to determinelocations corresponding to those locations, in which the stator 13 ispressurized.

Finally, pressurization is applied in predetermined locations on theouter periphery of the stator 13. The pressurizing portions 14 are madein agreement with the four predetermined locations on the outerperiphery of the stator 13 to be press fitted between the armour part 15and the stator.

Accordingly, the embodiment 9 has a feature comprising the step ofinserting and assembling the rotor 12 having permanent magnets of 8poles (P is a natural number) into the stator 13 formed to betorus-shape and having 12 slots, on which a coil is arranged, the stepof rotating the rotor 12 in a state, in which electric current is notcaused to flow through a coil of the stator 13, to measure coggingtorque every angle, the step of determining those locations, in whichthe outer periphery of the stator 13 is pressurized, on the basis ofmeasurements of cogging torque, and the step of assembling thepressurizing parts, which pressurize the outer periphery of the stator,to an outside of the stator in four locations, four being a plus minimumvalue calculated from N=4, N=±2×4−12, or N=12±2×4.

Thereby, since a state of cogging torque before the outer periphery ofthe stator 13 is pressurized in predetermined locations is made use of,the cause for generation of cogging torque can be dealt with as a blackbox and components caused by non-uniformity of cogging torque every slotcan be decreased in case of whatever cause, so that it is possible todecrease cogging without investigating the cause for cogging torque.

In addition, in the case where cogging torque of the same number as thenumber of poles of the rotor 12 is not generated in the step ofmeasuring cogging torque, a part in the form of an ordinary circularpipe for fixation of the stator 13 may be used to fix the stator 13without the use of the pressurizing portions 14 having a structure forpressurization and the frame 15. Also, the manufacturing methodaccording to the embodiment can be likewise used for the frame 15according to the embodiment 8.

EMBODIMENT 10

The embodiment 10 will be described with respect to a permanent-magnettype synchronous motor capable of decreasing cogging torque in the casewhere the number of locations, in which a stator is pressurized by apressurizing part, is not a minimum numeral among solutions of thecondition N.

FIG. 15 is a cross sectional view showing a permanent-magnet typesynchronous motor according to the embodiment 10. In contrast to theembodiment 6, a rotor has 4 poles and a stator has 12 slots in thepresent embodiment. Also, magnets 12 c in the embodiment are ideallyarranged to provide for a uniform and symmetrical magnetic flux densitydistribution. On the other hand, since the stator 13 includes nonuniformportions in terms of manufacture, pulsating components are contained inpermeance. An influence, which is produced on a product by the pulsatingcomponents, will be described later.

Also, at the time of measurement of cogging torque in thepermanent-magnet type synchronous motor, according to the embodiment 10,with 4 poles and 12 slots, components of 4 times being the same numberas the number of poles of the rotor 1 are in some cases contained perone revolution of the rotor 1.

While the embodiments 6 to 9 have been described with respect to thecase of a plus minimum value among the calculated solutions of N,cogging torque is in some cases decreased even in the case where asolution of N is not a minimum value, when components having an oppositephase to that of pulsating components of N times in the permeancedistribution function formed by the stator 13 are given. In 4 poles and12 slots in the embodiment 10, pulsating components of N times in thepermeance distribution function where N=4 is a plus value among valuescalculated from N=2, N=±2×2−12×0, or N=12×0±2×2 assuming that i1=0 inthe formula relating to the number N of pulsating components aregenerated.

In addition, while an explanation is given centering on i1>0 in theNon-Patent Document 1, even i1≧0 is established theoretically since i1is a spatial order when the permeance distribution function is expandedin Fourier series. When the formulae (2), (3) are established, thesmaller i1, the larger an amplitude of cogging torque.

As shown in FIG. 15, four pressurizing parts 14, four being equal to thetimes 4 of pulsating components contained in the permeance distributionfunction of the stator 13 are prepared, a point, at which cogging torqueis zero in measurement of cogging torque, or a neighborhood thereofmakes a first position of pressurization, and the remaining positions ofpressurization are arranged at equal angular intervals. Predeterminedstress and displacement are given to the stator 13 by press fitting thepressurizing parts between an armour frame 14 and the stator 13. Thestress changes the stator 13 locally in relative permeability.

A change, in relative permeability, caused by displacement and stress ofthe stator 13 results in pulsations of 4 times per one revolution of therotor 12 as an air gap length between the stator 13 and the rotor 12,and relative permeability also results in pulsations of 4 times per onerevolution of the rotor 12 due to stress of a back yoke of the stator13. Therefore, when the positional relationship of the stator 13 and therotor 12 is put in order, the permeance distribution function also givescomponents having an opposite phase to that of pulsating components of 4times.

In this case, it has been confirmed that cogging torque is decreasedbefore and after pressurization by the pressurizing parts 14.Accordingly, the permeance distribution function also generatespulsating components of 4 times, and pressurization by the pressurizingparts 14 makes it possible to give components having an opposite phaseto that of the pulsating components.

As described above, since the embodiment comprises the stator 13 having12 slots, on which a coil is arranged, the rotor 12 having permanentmagnets of 4 poles and inserted into a torus of the stator 13, and thepressurizing parts 14 that pressurize an outer periphery of the stator13 inward in N locations, N being a plus minimum value found from theformula (1) N=p, or the formula (2) N=±2 p−i1×Z, or the formula (3)N=i1×Z±2 p, stress and displacement by the stress are imparted to thestator 13 in predetermined locations to cancel pulsating components of 4times, in permeance, formed by the stator 13, thereby enablingdecreasing cogging torque having pulsating components of the same numberas the number of poles of the rotor 12.

With the embodiment, N=2 is obtained from the formula (1), N=±2×2−12×0=4is obtained from the formula (2), or N=12×0±2×2=4 is obtained from theformula (3). 4 being a plus value is adopted among these values of N,and pressurization is applied in four locations.

In addition, the same effect can be obtained even in the case where inplace of the pressurizing parts 14 and the armour part 15, the frame 15is press fitted into the stator 13 in the same manner as in theembodiment 8.

EMBODIMENT 11

The embodiment 11 will be described with respect to the case where apermanent-magnet type synchronous motor according to the embodiment 10is applied to the manufacturing method according to the embodiment 9.

An explanation is given to a difference in processes between theembodiment and the embodiment 9. In the process of determining thenumber of those locations, in which the stator 13 is pressurized, fromdata of cogging torque of the rotor 12 every angle, determination ismade by the use of a plus minimum numeral among solutions of N, whichmeet any one of N=p, N=±2 p−i1×Z, and N=i1×Z±2 p, in the embodiment 9,while the embodiment 11 is different from the embodiment 9 in that N isdetermined to be 4 in view of the case where pulsating components of N=4being a plus value, in the permeance distribution function aregenerated. A permanent-magnet type synchronous motor can be manufacturedby making subsequent processes the same as those in the embodiment 9.

Accordingly, since the embodiment 11 comprises the step of inserting andassembling the rotor 12 having permanent magnets of 4 poles (P is anatural number) into the stator 13 formed to be torus-shape and having12 slots, on which a coil is arranged, the step of rotating the rotor 12in a state, in which electric current is not caused to flow through thecoil 18, to measure cogging torque every angle, the step of determiningthose locations, in which the outer periphery of the stator 13 ispressurized, on the basis of measurements of cogging torque, and thestep of assembling the pressurizing parts 14, which pressurize the outerperiphery of the stator 13, to an outside of the stator 13 in fourlocations, four being a plus minimum value calculated from N=2,N=±2×2−12×0, or N=12×0±2×2, a state of cogging torque before the outerperiphery of the stator 13 is pressurized in predetermined locations ismade use of, so that the cause for generation of cogging torque can bedealt with as a black box and components caused by non-uniformity ofcogging torque every slot can be decreased in case of whatever cause,whereby it is possible to decrease cogging without investigating thecause for cogging torque.

In addition, in the case where cogging torque of the same number as thenumber of poles of the rotor 12 is not generated in the step ofmeasuring cogging torque, a part in the form of an ordinary circularpipe for fixation of the stator 13 may be used to fix the stator 13without the use of the pressurizing portions 14 having a structure forpressurization and the frame

1. A permanent-magnet synchronous motor comprising: a toroidal statorcore having an outer periphery, a plurality of internal radial teeth,and Z slots, each slot separating a pair of teeth, and a coil arrangedon the stator core, wherein Z is a natural number; a rotor havingpermanent magnets with 2 p poles, the rotor being disposed within thetoroidal stator core, wherein p is a natural number; and a pressurizingpart that applies a local extreme pressure to the outer periphery of thetoroidal stator core, radially inwardly, at each of N locations, whereinN is any one of positive values of values chosen from values calculatedfrom N=p, N=±2 p−Z(i1), and N=Z(i1)±2 p, i1 is an integer and at least0, and the N locations are chosen so that each local extreme pressureapplied is aligned with a center line of at least one of the radialteeth.
 2. The permanent-magnet synchronous motor according to claim 1,wherein N is the positive minimum value of the values calculated fromN=±2 p−Z(i1), and Z(i1)±2 p.
 3. The permanent-magnet synchronous motoraccording to claim 1, wherein the pressurizing part comprises a framethat fixes the stator core in position with respect to the frame androtatably supports one end of the rotor.
 4. A permanent-magnetsynchronous motor comprising: a toroidal stator core having an outerperiphery, a plurality of internal radial teeth, and Z slots, each slotseparating a pair of teeth, and a coil arranged on the stator core,wherein Z is a natural number; a rotor having permanent magnets with 2 ppoles, the rotor being disposed within the toroidal stator core, whereinp is a natural number; and a pressurizing part that applies a localextreme pressure to the outer periphery of the toroidal stator core,radially inwardly, at each of N locations, wherein N is any one ofpositive values of values chosen from values calculated from N=p, N=±2p−Z(i1), and N=Z(i1)±2 p, i1 is an integer and at least 0, and the Nlocations are chosen so that each local extreme pressure applied isaligned with a center line of at least one of the slots.
 5. Thepermanent-magnet synchronous motor according to claim 4, wherein N isthe positive minimum value of the values calculated from N=±2 p−Z(i1),and Z(i1)±2 p.
 6. The permanent-magnet synchronous motor according toclaim 4, wherein the pressurizing part comprises a frame that fixes thestator core in position with respect to the frame and rotatably supportsone end of the rotor.